1. Field of the Invention
The invention relates to targeted binding agents against KDR and uses of such agents. More specifically, the invention relates to fully human monoclonal antibodies directed to KDR. The described targeted binding agents are useful in the treatment of diseases associated with the activity and/or overproduction of KDR and as diagnostics.
2. Description of the Related Art
The vascular endothelial growth factor-A (VEGF-A) plays a critical role in inducing vascular growth and remodeling during development, and in a number of pathological conditions including the angiogenesis required to support solid tumour growth. VEGF-A signaling is predominantly mediated through activation of VEGF receptor 2 (VEGFR2; KDR/flk-1), which can stimulate endothelial cell proliferation, migration, vascular permeability, and neovascular survival (reviewed in Olsson et al., Nat. Rev. Mol. Cell. Biol. 2006; 7:359-71.). The broader VEGFR family of tyrosine kinase receptors consists of three members: VEGFR1, VEGFR2 and VEGFR3, also known as Flt-1, KDR/Flk-1 and Flt4, respectively. Known VEGFR ligands exhibit differential, well-defined selectivity for each VEGFR. For example, VEGF-A binds both VEGFR1 and VEGFR2, VEGF-B and P1GF primarily bind VEGFR1, and VEGF-C and D are specific activators of VEGFR3. There are also a number of non-physiological VEGFs that also activate the VEGFR, for example VEGF-E specifically activates VEGFR2 while VEGF-F will activate VEGFR1 and VEGFR2.
Dimerization of VEGFR drives a complex series of signaling events leading to activation of many common growth factor signaling pathways (Olsson et al Nat. Rev. Mol. Cell Biol. 2006; 7:359-71). At the molecular level, there is a complex relationship between ligand expression, receptor dimerization and activation, and the downstream consequences in different cell types. VEGFR signaling is of primary importance to endothelial cells, although the receptors have been implicated in regulating the function of other cells. For example Flt-1 plays a role in mediating monocyte transmigration and, when expressed, can promote tumour cell migration.
All three receptors form both homo- and heterodimers (with the exception of VEGFR1/3 heterodimers), allowing signals from the various VEGF ligands to be integrated. KDR appears to be the receptor that is central to many of these signaling events, as it is the common VEGFR expressed on blood and lymphatic vessels. KDR can also offset low VEGFR-1 signaling activity. VEGFR1 itself has low intrinsic kinase activity, and deletion of the VEGFR1 kinase domain does not affect normal development. Experiments have shown that VEGFR1 can synergize with KDR and facilitate full activation of KDR signaling (Carmeliet et al, Nat Med 2001, 7, 575; Auterio et al, 2003 Nat Med, 9, 936). It is possible that this is achieved through heterodimerisation, which is prevented by an inhibitor of KDR dimerization. VEGFR3 has also been shown to form a heterodimer with KDR (Alam et al, BBRC, 2004, 324, 909). However, the mechanisms by which the various homo- and heterodimers of VEGFR1, KDR and VEGFR3 interact to drive physiological effects is unclear. Inhibition of VEGFR/VEGF signaling may effect different disease states (reviewed in Baka et al Expert Opin Ther Targets 2006, 10, 867).
VEGFRs consist of seven immunoglobin-like extracellular domains. Ligands, (e.g. VEGFA and P1GF) that specifically bind VEGFRs have immunoglobin-like domains 2 and 3, with domain 2 making the primary contact and domain 3 determining the specificity of binding (Christinger et al JBC, 2004, 279, 10382; Fuh et al JBC 1998, 273, 11197). In contrast, domains 4-6 are involved in dimerization of the receptor complexes. Ligand binding that stabilizes the receptor complexes can prolong dimerization, allowing productive signaling to proceed. As receptor activation is a function of both binding and dimerization, receptor activation can be inhibited by inhibiting ligand-receptor binding or by blocking dimerization. Antibodies that block binding of ligand to the receptors have been described previously, for example, IMC1121b and its murine equivalent, DC101, both of which block binding of VEGF-A to KDR, and thereby block VEGF-A signaling through KDR, and are known to deliver an anti-tumour effect (Prewett et al Cancer Res 1999, 59, 5209; Lu et al, JBC 2003, 278, 43496). It has been reported both pre-clinically and clinically that as a direct consequence of inhibiting KDR signaling, there is a rebound increase in circulating VEGF-A levels. Antibodies such as IMC1121b which block VEGF-A binding to KDR are expected to be less efficacious under conditions of increasing VEGF-A concentrations, where competition for binding to KDR may be won over by VEGF-A rather than the antibody.
Thus there is a need to identify new means of inhibiting KDR signaling.